Add basic docs for (Lazy)Artifacts stdlibs (#39073)

(cherry picked from commit debf26e)

Author: fingolfin

stdlib/Artifacts/docs/src/index.md

@@ -0,0 +1,21 @@
+# Artifacts
+
+```@meta
+DocTestSetup = :(using Artifacts)
+```
+
+Starting with Julia 1.6, the artifacts support has moved from `Pkg.jl` to Julia itself.
+Until proper documentation can be added here, you can learn more about artifacts in the
+`Pkg.jl` manual at <https://julialang.github.io/Pkg.jl/v1/artifacts/>.
+
+!!! compat "Julia 1.6"
+    Julia's artifacts API requires at least Julia 1.6. In Julia
+    versions 1.3 to 1.5, you can use `Pkg.Artifacts` instead.
+
+
+```@docs
+Artifacts.artifact_meta
+Artifacts.artifact_hash
+Artifacts.find_artifacts_toml
+Artifacts.@artifact_str
+```

stdlib/Artifacts/src/Artifacts.jl

@@ -551,15 +551,15 @@ function _artifact_str(__module__, artifacts_toml, name, path_tail, artifact_dic
     error("Artifact $(repr(name)) was not installed correctly. Try `using Pkg; Pkg.instantiate()` to re-install all missing resources.")
 end
 
-"""
+raw"""
     split_artifact_slash(name::String)
 
 Splits an artifact indexing string by path deliminters, isolates the first path element,
 returning that and the `joinpath()` of the remaining arguments.  This normalizes all path
 separators to the native path separator for the current platform.  Examples:
 
 # Examples
-```jldoctest
+```jldoctest; setup = :(using Artifacts: split_artifact_slash)
 julia> split_artifact_slash("Foo")
 ("Foo", "")
 

stdlib/LazyArtifacts/docs/src/index.md

@@ -0,0 +1,10 @@
+# Lazy Artifacts
+
+```@meta
+DocTestSetup = :(using LazyArtifacts)
+```
+
+In order for a package to download artifacts lazily, `LazyArtifacts` must be
+explicitly listed as a dependency of that package.
+
+For further information on artifacts, see [Artifacts](@ref).